I always have it too. It seems zoanthids are such a picky group sometimes. First you notice something awry and think leave them alone for a day or so, then they have junk growing on them and go downhill fast. I have been able to save many zoanthids by dipping. They always come right back and the ones that are pretty far gone might require two or three treatments. It is important to give them an opportunity to come back after treatment. Three treatments in a row, max, and then a week off for the dipees. On the other hand, I have had zoas that looked like crap so I threw them in a corner of the tank and find them in great shape a year later. I think I am part of the problem because I don't run a heater in the summer and get temperature changes that are likely a bit much for zoanthids.